3GPP SA2 has defined an architecture for the fifth Generation System, 5GS, and Evolved Packet System, EPS, interworking based on a number of combined functions. For migration purposes, a Home Subscriber Server+Unified Data Management, HSS+UDM, combination is introduced, and defined as one of the combination functions specified to support interworking between Evolved Packet Core, EPC, and 5GC networks.
If a particular User Equipment, UE, moves from 5G to EPS, then it sends an initial attach/TAU Request to the Mobility Management Entity, MME, that requests the UE context from the Access and Mobility Management function, AMF, where the UE was/is camping. Such a request may involve the N26 interface between the AMF and the MME. After the authentication process, the MME needs to update the location of the UE in the HSS, as is traditionally done in EPC.
After that, the HSS+UDM combination needs to execute UDM business logic and needs to de-register the former AMF. It informs the AMF that it has been de-registered, and the AMF then unsubscribes from subscription data modification changes.
The present disclosure is also directed to the inverse scenario, in which a particular UE was registered in EPC and moves to 5GC. Then, the UE first sends a registration request to the AMF. After the authentication procedure, the new AMF has to be registered to the HSS+UDM combination using the UDM business logic, and services. It will fetch the corresponding subscription data and subscribes to be notified when the subscription data is modified.
Then, the HSS business logic needs to send a Cancel Location messages to the former MME, since the UE is kept registered in one serving node only.
One of the drawbacks of the above described situations, is that there is only limited flexibility in design choices for the HSS and the UDM.